As the interrupting and overload switching requirements of circuit breakers increase, contact erosion usually increases. Arcing erodes the circuit breaker contact material shortening the contact life. Contact erosion may be reduced by supplying each phase of the circuit breaker with arcing contacts in addition to the main current carrying contacts. Arcing contacts are made of an arc resistant material such as silver tungsten. The arcing contacts are the first contacts to mate upon the breaker closing and the last contacts to separate upon the breaker opening. Thus the arc is drawn between the arcing contacts.
The arc resistant material of the arcing contacts usually has a relatively high resistance. The arcing erosion of the contacts increases that resistance. Separate main contacts are relied upon to carry the current when the circuit breaker is in the closed position. The main contact material, such as silver cadmium oxide generally has a lower resistance than that of the arcing contacts but is also less resistant to arcing erosion than the arcing contact material. The conventional configuration for a contact assembly in a single pivot circuit breaker is shown in FIG. 1. This type of circuit breaker often has fixed lower contacts and pivoted moving contacts. The moving arcing contact blade is lonqer than the moving main contact blades to keep the arc away from the main contacts and facilitate its transfer to the stack.
As customers began reguiring higher withstand capabilities of circuit breakers, the constriction forces between the contacts could cause the contacts to blow open below the desired withstand level. The single pivot circuit breaker designs were replaced with double pivot circuit breakers that have a blow on loop as shown in FIG. 2. The opposing path of the current in the line terminal 102 and semi-stationary contact 103 causes the semi-stationary contact to move upwards. Pressing against the moving contacts. As the current increases, the blow on loop force increases. giving the breaker the capability to withstand higher fault currents.
The longer arcing contact blade of the single pivot is not easily used with a double pivot configuration since it requires many additional components and is not cost effective.
The double pivot design often utilizes main contact blades 106 and arcing contact blades 108 of the same length, as shown in FIG. 3. An arcing horn 110 extends beyond the center arcing contact to provide an arc path away from the contacts. This design suffers excessive arcing on the outer main contacts especially during multi-phase faults at the overload switching levels.
There is a need for a contact assembly adaptable for use with a double pivot circuit breaker to quickly transfer the arc off the arcing or main contacts.
There is a further need for a contact assembly adaptable for use for a double pivot circuit breaker to more; effectively elongate and cool the arc in the arc stack.
These and other features of this invention will become more readily apparent from the following description, claims and drawings.